Human populations are now faced with the genetic as well as somatic hazards by being exposed to an increasing amount of irradiations and medicines or chemicals. One of the main causes of such hazards is mutational, or its resultant (i.e., recombination with other genes) events, and the present research is aimed to resolve the urgent problems in genetics--the mechanisms involved in the mutation and/or recombination processes, and further to resolve the genetic mechanisms which regulate the rates of occurrences of these events. It is also aimed to establish the genetic basis of systems which regulate the Mendelian ratios. The male recombination-mutator system in Drosophila melanogaster is one of the best systems in eukaryotes suitable for this study, since it affects all three genetic phenomena described above. Using this material, the above problems will be studied at the levels of formal genetics, cytogenetics, and population genetics. Studies in cytogenetics will hopefully open the door through which we can study this problem at the molecular level. Studies in male recombination system would be further extended to include Segregation Distorter system found in the same species, and studies on these two systems together will provide the most valuable information for the present problems at various levels of approach.